Carbon fiber bundle that develops high mechanical performance

ABSTRACT

Provided is a carbon fiber bundle for obtaining a fiber-reinforced resin having high mechanical characteristics. A carbon fiber bundle formed of single carbon fibers, each of which has no uneven surface structure of 0.6 μm or more in length extending in the longitudinal direction of the single fiber; which has an uneven structure having a difference in height (Rp−v) of 5 to 25 nm between the highest portion and the lowest portion of the surface of the single fiber and having an average roughness Ra of 2 to 6 nm; and which has a ratio of the major axis to the minor axis (major axis/minor axis) of a cross-section of the single fiber of 1.00 to 1.01, wherein a mass of the single fiber per unit length falls within the range of 0.030 to 0.042 mg/m; a strand strength is 5900 MPa or more; a strand elastic modulus measured by the ASTM method is 250 to 380 GPa; and a knot tenacity is 900 N/mm 2  or more.

TECHNICAL FIELD

The present invention relates to a carbon fiber bundle that hasexcellent mechanical characteristics and that is particularly used forobtaining a fiber-reinforced resin using a high-tenacity heat resistantresin as a matrix for use in the construction of airplanes.

BACKGROUND ART

Conventionally, in order to improve the mechanical characteristics ofresin-base molded products, a resin has been commonly used incombination with a fiber serving as a reinforcement material. Inparticular, a composite molding material formed of a carbon fiber thatis excellent in specific strength and specific elasticity in combinationwith a high-performance resin develops extremely high mechanicalcharacteristics. Because of this, such a molding material has beenwillingly used as a constructional material for airplanes, high speedmoving bodies, etc. Furthermore, there is a demand for developing amaterial that is stronger and that has higher rigidity as well as havingexcellent specific strength and specific rigidity. Given thesecircumstances, the desire is for further improvement of the performanceof carbon fiber, such as improved strength and elastic modulus.

For example, Patent Literature 1 proposes a method of drawing acoagulated fiber that still contains a solvent in a solvent-containingdrawing bath, thereby improving uniformity in structure and orientation,in order to obtain an acrylic fiber bundle used as a precursor of acarbon-fiber by a dry-wet spinning method. Drawing a coagulated fiber ina bath containing a solvent is a method commonly known as a solventdrawing technique that enables a stable drawing process by using solventplasticization. Accordingly, this method is considered as an extremelyexcellent technique for obtaining a fiber that has high uniformity instructure and orientation. However, if a fiber bundle that is in aswollen state due to the presence of a solvent is drawn, the solventwithin a filament is rapidly squeezed out from the filamentsimultaneously upon drawing. The resultant structure of the filamenttends to be less dense and thus a desired filament that has a densestructure cannot be obtained.

Furthermore, Patent Literature 2, which pays attention to fine poresdistributed in a coagulated fiber, proposes a technique for obtaining aprecursor fiber in which excellent strength is developed bydry-densification of a coagulated fiber that has a high-dense structure.The fine pore distribution, which is obtained by a mercury press-inmethod, reflects the bulk state from the surface layer to the interiorof the filament. This is an extremely excellent method for evaluatingthe overall density of a fiber structure. From the precursor fiberbundle that has at least a certain level density as a whole, a verystrong carbon fiber can be obtained in which defect point formation issuppressed. However, observation of fractures in the carbon fiber showsthat fractures have originated from near the surface layer at anextremely high ratio. This means that a defective point is present nearthe surface layer. In other words, this technique is insufficient formanufacturing a precursor fiber bundle that is excellent in density nearthe surface layer.

Patent Literature 3 proposes a method for manufacturing anacrylonitrile-based precursor fiber bundle that is not only high inwhole density but that is also extremely high in surface density.Furthermore, Patent Literature 4 proposes, taking into considerationthat an oil solution enters the surface-layer portion of a fiber andinhibits densification, a technique for suppressing permeation of an oilsolution by focusing on microscopic voids of the surface-layer portion.However, a technique for suppressing the entry of an oil solution and atechnique for suppressing defective point formation are both difficultto put into practical use since very complicated steps are required.Therefore, in the techniques discussed above, the effect of stablysuppressing the entry of an oil solution into the surface layer portionis insufficient and the effect of reinforcing a carbon fiber is stillfar from a sufficient level.

CITATION LIST Patent Literatures

-   Patent Literature 1: JP05-5224A-   Patent Literature 2: JP04-91230A-   Patent Literature 3: JP06-15722A-   Patent Literature 4: JP11-124744A

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a carbon fiber bundlefor obtaining a fiber-reinforced resin that has high mechanicalcharacteristics.

Solution to Problem

The object is attained by the invention set forth below.

The present invention is directed to a carbon fiber bundle formed ofsingle carbon fibers, each of which has no surface uneven structure of0.6 μm or more in length extending in the longitudinal direction of thesingle fiber; which has an uneven structure having a difference inheight (Rp−v) of 5 to 25 nm between the highest portion and the lowestportion of the surface of the single fiber and an average roughness Raof 2 to 6 nm, and which has a ratio of the major axis to the minor axis(major axis/minor axis) of a cross-section of the single fiber of 1.00to 1.01, in which a mass of the single fiber per unit length fallswithin the range of 0.030 to 0.042 mg/m, a strand strength is 5900 MPaor more, a strand elastic modulus measured by the ASTM method is 250 to380 GPa and a knot tenacity is 900 N/mm² or more.

Note that the knot tenacity can be obtained by dividing the tensilebreaking stress of a knotted carbon fiber bundle by the cross-sectionalarea of the fiber bundle (mass and density per unit length).

Advantageous Effects of Invention

According to the carbon fiber bundle of the present invention, it ispossible to provide a fiber-reinforced resin that has high mechanicalcharacteristics.

Furthermore, a carbon fiber bundle that has even better performance isobtained by acquiring “surface energy of surface formed by fracture”such that it reaches 30N/m or more.

Moreover, a carbon-fiber composite material that has extremely highmechanical performance can be obtained by forming a carbon fiber bundlewhich has an ipa value of 0.05 to 0.25 μA/cm² that is obtained by anelectrochemical measuring method (cyclic voltammetry), and in which theamount of an oxygen-containing functional group (O1S/C1S) in a carbonfiber surface, that is obtained by X-ray photoelectron spectroscopy, iswithin the range of 0.05 to 0.10.

DESCRIPTION OF EMBODIMENTS

The uneven surface structure present on the surface of a carbon fiberand extending in the longitudinal direction thereof and a sizing agentattached onto the surface play very important roles in developingmechanical characteristics of a fiber-reinforced resin material thatuses the carbon fiber as a reinforcement material. This is because theuneven surface structure and the sizing agent attached onto the surfaceare directly involved in formation of the interfacial phase between acarbon fiber and a resin as well as characteristics thereof. Themechanical performance of the fiber-reinforced resin material isinfluenced by the performance of each of the three constituent factors,i.e., the fiber, the matrix resin and the interfacial phase. Even ifonly one of the three factors is unsatisfactory, the fiber-reinforcedresin material cannot develop excellent mechanical performance.

(Uneven Surface Structure of a Single Fiber Extending in theLongitudinal Direction)

A carbon fiber obtained by a general manufacturing method for a carbonfiber bundle generally has an uneven surface structure that is formedalmost parallel to the fiber-axis direction. The uneven structure has anundulation structure that is almost parallel to the fiber axis and thatextends in the fiber axis. The depth of the uneven structure is usuallyabout 50 nm to several hundreds of nm and the length thereof is usuallyabout 0.6 μm to several μm and sometimes, several tens of μm. Such anuneven surface structure is usually called as surface wrinkle.

The carbon fiber bundle of the present invention does not have an unevensurface structure that has a length of 0.6 μm or more and that extendsin the longitudinal direction of a single fiber.

In contrast, the carbon fiber bundle of the present invention has anuneven structure that is smaller than the uneven structure mentionedabove on the surface of a single fiber. The depth of the uneven surfacestructure present on a single carbon fiber is defined by the differencein height (Rp−v) between the highest portion and the lowest portion onthe surface of a fiber and the average roughness Ra, in an area rangesurrounded by a length of 1.0 μm in the fiber-circumference directionand a length 1.0 μm in the fiber-axis direction. The (Rp−v) and Ra canbe obtained by scanning the surface of a single fiber by use of ascanning atomic force microscope (AFM). It is desirable that thedifference in height (Rp−v) be 5 to 25 nm and that the average roughnessRa be 2 to 6 nm. It is more preferable that the difference in height(Rp−v) be 5 to 18 nm and that Ra be 2 to 5 nm.

In the present invention, each of the single fibers that constitutes acarbon fiber has no uneven surface structure that has a length of 0.6 μmor more and that extends in the longitudinal direction of the fiber, onthe surface of a single fiber. In the interfacial phase of a compositematerial, stress tends to be concentrated on such a large uneven surfacestructure. In addition, the fracture toughness of carbon fiber tissuearound such an uneven structure is low. Accordingly, in the unevensurface structure of this size, interfacial failure tends to originatefrom a point near the uneven structure even if the level of stressapplied to a composite material is not very large. As a result,mechanical performance of the composite material significantlydecreases.

A more specific embodiment of the uneven surface structure of each ofthe single fibers constituting a carbon fiber bundle of the presentinvention is as follows.

Usually, carbon-fiber surface has an uneven structure of 0.6 μm or morein length called a wrinkle structure, that has an assembly of severalfibrils as a unit and that extends in the longitudinal direction of thefiber, and an uneven microscopic structure that is smaller than theuneven structure called a wrinkle structure and that is present in eachfibril body itself.

On the contrary, on the surface of each of the single fibersconstituting a carbon fiber bundle of the present invention, an unevenstructure that has a length of 0.6 μm or more and that extends in thelongitudinal direction of a fiber is not present but only an unevenmicroscopic structure that is smaller than the uneven structure and thatis present in each fibril body itself is present. Furthermore, such anuneven microscopic structure has a length of 300 nm or less. The unevenstructure is defined by (Rp−v) and Ra as mentioned above. To bespecific, the uneven structure is an undulation structure having adifference in height (Rp−v) of 5 to 25 nm and an average roughness Ra of2 to 6 nm, which are present in an area range surrounded by a length(1.0 μm) in the fiber-circumference direction of a single-fiber surfaceand a length (1.0 μm) in the fiber-axis direction. Preferably, (Rp−v) is5 to 18 nm and Ra is 2 to 5 nm. The direction of the uneven microscopicstructure is not particularly limited and may be parallel orperpendicular to the fiber-axis direction or may be present at an anglewith the fiber-axis direction.

(Cross-Section of Single Fiber)

Furthermore, the ratio of the major axis and the minor axis (majoraxis/minor axis) of a single fiber cross-section is 1.00 to 1.01. Thismeans that the single fiber must have a complete circular or nearlycomplete circular cross-section. This is because if the cross-section isa complete circle, the portion near the fiber surface has excellentstructural uniformity, with the result that concentration of stress canbe reduced. The ratio is preferably 1.00 to 1.005. Furthermore, for thesame reason, the mass of the single fiber per unit length is 0.030 to0.042 mg/m. If the mass of the single fiber per unit length (singlefiber weight per unit area) is low, the fiber diameter is small, andstructural irregularity along the cross-section is small. This meansthat mechanical performance in the direction perpendicular to the fiberaxis is high. Accordingly, in a composite material, tolerance to stressapplied in the direction perpendicular to the fiber axis is improved andthe mechanical performance of a composite material can be enhanced.

(Carbon Fiber Bundle)

In the present invention, to obtain a fiber-reinforced resin havingexcellent mechanical characteristics, the strand strength of a carbonfiber bundle must be 5900 MPa or more. The strand strength of a carbonfiber bundle is preferably 6000 MPa or more and more preferably 6100 MPaor more. The higher the strand strength, the better; however, taken intoconsideration the balance between the strand strength and thecompressive strength of a composite material, 10000 MPa is sufficient.Furthermore, in the present invention, to obtain a fiber-reinforcedresin having excellent mechanical characteristics, the strand elasticmodulus of a carbon fiber bundle must be 250 to 380 GPa, which arevalues measured by the ASTM method. If the elastic modulus is less than250 GPa, the elastic modulus of a carbon fiber bundle will beinsufficient and sufficient mechanical characteristics cannot develop.In contrast, if the elastic modulus exceeds 380 GPa, the graphitecrystal size of the surface and interior of a carbon fiber willincrease. In accordance with this, the strength along the cross-sectionof the fiber and the compressive strength in the fiber-axis directiondecrease and performance balance between tension and compression of acomposite material cannot be maintained. As a result, excellentcomposite material cannot be obtained. In addition, inactivation of thesurface proceeds as the graphite crystal size increases, and theadhesiveness with a matrix resin decreases. As a result, mechanicalperformance such as tensile strength of the composite material in thedirection of 90°, interlayer shearing strength, in-plane shearingstrength and 0° compressive strength significantly decreases.

Furthermore, in the present invention, it is important that the knottenacity, which is obtained by dividing the tensile stress at break of aknotted carbon fiber bundle by the cross-sectional area of the fiberbundle (mass and density per unit length), is 900 N/mm² or more. Morepreferably, the knot tenacity is 1000 N/mm² or more and furtherpreferably, 1100 N/mm² or more. The knot tenacity serves as an indexthat reflects the mechanical performance of a fiber bundle except in thefiber-axis direction. In particular, the performance in the directionperpendicular to the fiber axis can be simply checked by the knottenacity. In the composite material, since a material is often formed bypseudo-isotropic lamination, a complicated stress field is formed. Atthis time, other than tension and compression stress in the fiber-axisdirection, stress is also generated in the fiber-axis direction.Furthermore, if a relatively high-speed strain is applied, as is in animpact test, the state of the stress that is generated within thecomposite material is considerably complicated and the strength in thedirection that is different from the strength in the fiber-axisdirection becomes important. Accordingly, if the knot tenacity is lessthan 900 N/mm², sufficient mechanical performance does not develop in apseudo-isotropic material. On the other hand, if the knot tenacityexceeds 3000 N/mm², the degree of orientation in the fiber-axisdirection must be reduced. Accordingly, the knot strength should becontrolled to be 3000 N/mm² or less.

Furthermore, in the carbon fiber bundle of the present invention, a“surface energy of surface formed by fracture” is preferably 30 N/m ormore. The surface energy of surface formed by fracture is obtained byforming a hemispherical defect having a predetermined size by a laser onthe surface of a single fiber, and by breaking the fiber at thehemispherical defective site in a tensile test and by calculating fromthe breaking strength of the fiber and the size of the hemisphericaldefect in accordance with the following Griffith Equation (1).

σ=(2E/πC)^(1/2)×(surface energy of surface formed byfracture)^(1/2)  (1)

Here, “σ” is the breaking strength; “E” is the ultrasonic elasticmodulus of a carbon fiber bundle; and “C” is the size of a hemisphericaldefect. The “surface energy of surface formed by fracture” is morepreferably, 31 N/m or more and further preferably 32 N/m or more.

The surface energy of surface formed by fracture herein is used as abreak-proof index of a carbon fiber for representing substrate strength.The carbon fiber is a material showing brittle fracture and the tensilestrength is controlled by the defective point. If carbon fibers have thesame defective points, the breaking strength increases as the substratestrength increases. Furthermore, a matrix resin for a high-performancecomposite material often has high adhesiveness with a carbon fiber, withthe result that the critical fiber length, which serves as an index forstress transfer, diminishes. As a result, the strength in a furthershorter length is reflected in the strength of the composite material.Thus, the substrate strength is conceivably an important index. Incontrast, if the surface energy produced by breaking exceeds 50 N/m, itis necessary to reduce the degree of orientation in the fiber-axisdirection. Accordingly, the surface energy produced by breaking shouldbe 50 N/m or less.

In the present invention, the ipa value obtained by an electrochemicalmeasuring method (cyclic voltammetry) is preferably 0.05 to 0.25 μA/cm².The ipa value is influenced by the number of oxygen-containingfunctional groups of a carbon fiber, surface roughness involved information of an electric double layer and the microscopic graphitestructure of a carbon fiber surface. In particular, a carbon fiber whosesurface is largely etched and a carbon fiber that has an intercalationcompound which is formed by inserting an anion between layers of agraphite crystal have a large ipa value. In developing compositematerial that has excellent mechanical performance, the interfacebetween a carbon fiber and a resin is important. In particular, a carbonfiber that has a surface in which an appropriate oxygen-containingfunctional group is present and in which a small electric double layeris formed, is found to form the most appropriate interface. If an ipavalue is 0.05 μA/cm² or more, it indicates that the surface has asufficient number of oxygen-containing functional groups, and itexhibits sufficient adhesiveness to an interface. In contrast, if an ipavalue is 0.25 μA/cm² or less, the surface is not excessively etched andan intercalation compound is not formed. Such a surface can tightlyadhere to a matrix resin, with the result that the surface cansufficiently adhere to the interface with a resin. The ipa value is morepreferably 0.07 to 0.20 μA/cm² and further preferably 0.10 to 0.18μA/cm².

Furthermore, in the present invention, it is desirable that the amountof an oxygen containing functional group in a carbon fiber surface(obtained by X-ray photoelectron spectroscopy) be within the range of0.05 to 0.15. This is because it is important to have an appropriateadhesiveness with a matrix resin at the interface.

Furthermore, in the present invention, a Si amount measured by ICPemission spectrometry is desirably 200 ppm or less. In order tomanufacture a high-strength carbon fiber, usually an oil solutioncontaining silicone oil is attached onto a precursor fiber bundle.Silicone oil has extremely excellent heat resistance properties and canimpart excellent mold release characteristics. Thus, silicone oil isconsidered most suitable as an oil solution for a carbon-fiber precursorbundle, which is a multifilament bundle formed by assembling a largenumber of filaments each having an extremely small diameter and eachbeing further subjected to a high-temperature treatment performed at atemperature of 200° C. or more for several tens of minutes to severalhours. However, in carbonization treatment performed after stabilizationtreatment, the silicone oil is mostly decomposed and scattered.Consequently, the amount of silicone compound remaining on the surfaceof a carbon fiber becomes extremely low. Furthermore, the remainingsilicone compound, which is present near the surface of a carbon fiber,has been found to contribute to the formation of voids. Therefore, ifthe amount of such a silicone compound is reduced as much as possible,carbon fiber having few voids can be manufactured. As a result, thestrength of a carbon fiber bundle can be increased. The more preferableSi amount is 150 ppm or less and a further preferable Si amount is 100ppm or less.

(Precursor Fiber Bundle and Manufacturing Method Thereof)

The starting material for obtaining a carbon fiber bundle of the presentinvention is not particularly limited; however, the starting materialthat can be obtained from an acrylonitrile-based precursor fiber(hereinafter, appropriately referred to as “precursor fiber”) ispreferred from the viewpoint of developing mechanical performance.

The acrylonitrile-based copolymer constituting the precursor fiber isobtained from acrylonitrile (96 mass % or more) and several types ofcopolymerizable monomers. More preferably, the composition ratio ofacrylonitrile is 97 mass % or more. Examples of copolymer componentsexcept acrylonitrile that are properly used include acrylic acidderivatives such as acrylic acid, methacrylic acid, itaconic acid,methyl acrylate and methyl methacrylate; and acryl amide derivativessuch as acryl amide, methacryl amide, N-methyolacrylamide andN,N-dimethyl acrylamide and vinyl acetate. These may be used alone or incombination. A preferable copolymer is an acrylonitrile-based copolymerobtained by copolymerization of a monomer that has one or more carboxylgroups as an essential component.

As an appropriate method for copolymerizing a monomer mixture, anypolymerization method may be used, including, for example, redoxpolymerization performed in an aqueous solution, suspensionpolymerization performed in non-homogeneous system and emulsionpolymerization using a dispersant. Difference between the polymerizationmethods does not limit the present invention. The precursor fiber ispreferably manufactured by dissolving the aforementioned acrylonitrilebased polymer in an organic solvent such as dimethyl acetamide, dimethylsulfoxide or dimethyl formamide to prepare dope. Since these organicsolvents do not contain a metal component, the content of a metalcomponent in the resultant carbon fiber bundle can be reduced. The solidsubstance concentration of the dope is preferably 20 mass % or more andmore preferably 21 mass % or more.

The spinning method may be either wet spinning or dry-wet spinning. Morepreferably, dry-wet spinning is employed. In dry-wet spinning, the dopethat is prepared is at first spun from a spinneret having numerousejection holes arranged therein into the air and is then introduced intoa congealed liquid filled with a solution mixture of an organic solventand water whose temperature is controlled so that the dope willcoagulate. The coagulated fiber is taken out, washed and drawn. As awashing method, any method may be used as long as the solvent can beremoved. Note that, before the coagulated fiber that has been taken outis washed, if drawing is performed in the pre-drawing tank that containsa solvent whose concentration is lower than in congealed liquid andwhose temperature is higher than that of congealed liquid, a fibrilstructure can be formed. When a coagulated fiber is drawn, thetemperature of the drawing tank preferably falls within the range of 40to 80° C. If the temperature is less than 40° C., a stable drawingcannot be secured and it causes the drawing to be poor, with the resultthat a uniform fibril structure cannot be formed. In contrast, if thetemperature exceeds 80° C., excessively strong plasticizing actionoccurs by heat, and a solvent is rapidly removed from a fiber surfaceand the result is nonuniform drawing. Therefore, the quality of theprecursor fiber bundle deteriorates. A more preferable temperature is 50to 75° C. Furthermore, the concentration of the drawing tank ispreferably 30 to 60 mass %. This is because if the concentration is lessthan 30 mass %, a stable drawing cannot be secured; whereas, if theconcentration is beyond 60 mass %, an excessively large plasticizationeffect is obtained and interference with a stable drawing occurs. A morepreferable concentration is 33 to 55 mass %.

The draw ratio in the drawing tank is preferably 2 to 4 times. If thedraw ratio is less than twice, drawing is insufficient and a desiredfibril structure cannot be formed. In contrast, if the draw ratio isbeyond 4 times, a fibril structure itself is broken. The result is aprecursor fiber bundle that has extremely low density. The draw ratio ismore preferably 2.2 to 3.8 times and further preferably, 2.5 to 3.5times.

Furthermore, after washing, if a fiber bundle that is processed in aswollen state without solvent is drawn in hot water, the orientationdegree of the fiber can be further enhanced. If a relaxing step isslightly performed, distortion caused by drawing in the previous stepcan be removed. Preferably, in order to improve the orientation degreeof a fiber by increasing the total draw ratio, drawing is performed inhot water at a ratio of 1.1 to 2.0 times.

Next, an oil solution composed of a silicone-based compound is attachedso as to apply 0.8 to 1.6 mass % and the resultant fiber is subjected todry densification. The dry densification is not particularly limited aslong as dry densification is performed by a known drying method.Preferably, a method of passing a fiber through a plurality of heatedrolls is employed.

The acrylic fiber bundle after the dry densification process has beenperformed, is, if necessary, drawn in pressurized steam of 130 to 200°C., in a dry heat medium of 100 to 200° C., between heated rolls of 150to 220° C. or on a heat board up to 1.8 to 6.0 times. After theorientation degree is further improved and densification is performed,the fiber bundle is rolled up to obtain a precursor fiber bundle.

Furthermore, a carbon fiber bundle of the present invention can bemanufactured from the above-mentioned precursor fiber bundle as follows.The precursor fiber bundle is fed to an oven for stabilization that is atype of furnace that circulates hot air at a temperature of 220 to 260°C. for 30 to 100 minutes to obtain a stabilized fiber having a densityof 1.335 to 1.360 g/cm³. At this time, an operation for extending thefiber by 0 to 10% is performed. The stabilization reaction consists of acyclization reaction with heat and an oxidation reaction with oxygen. Itis important to balance the two reactions. In order to balance the tworeactions, time for performing stabilization treatment is preferably 30to 100 minutes. If the reaction time is less than 30 minutes, a singlefiber has a portion in which the oxidation reaction does notsufficiently proceed, within the fiber, with the result that a largestructural variety is generated along the cross-section of the singlefiber. As a result, the obtained carbon fiber has a structure that isnon-uniformly formed and fails to develop high mechanical performance.In contrast, if the reaction time exceeds 100 minutes, a larger amountof oxygen will be present near the surface of a single fiber.Thereafter, in the following heat treatment performed at a hightemperature, a reaction that consumes an excessive amount of oxygenoccurs to form a defective point. Because of this, high strength cannotbe obtained. A more preferable stabilization treatment time is 40 to 80minutes.

In the case where the density of a stabilized fiber is less than 1.335g/cm³, stabilization treatment is insufficient. In the following heattreatment performed at a high temperature, a decomposition reactionoccurs to form a defective point. Because of this, high strength cannotbe obtained. If the density of a stabilized fiber exceeds 1.360 g/cm³,the oxygen content of the fiber increases. In the following heattreatment performed at a high temperature, a reaction that consumes anexcessive amount of oxygen occurs to form a defective point. Because ofthis, high strength cannot be obtained. A more preferable density rangeof a stabilized fiber is 1.340 to 1.350 g/cm³.

Appropriate extension of a fiber performed in an oven for stabilizationis required in order to maintain and improve the orientation degree of afibril structure that constitute a fiber. If the extension is less than0%, the orientation degree of a fibril structure cannot be maintained;the orientation degree along the fiber axis is not sufficient forforming a carbon fiber structure; excellent mechanical performancecannot develop. In contrast, if the extension exceeds 10%, the fibrilstructure itself will be broken, with the result that formation of acarbon fiber structure will be impaired and the fracture point willchange to a defective point. As a result, a carbon fiber that has highstrength cannot be obtained. A more preferable extension rate is 3 to8%.

Next, a stabilized fiber is passed through a first carbonization furnaceof an inert atmosphere such as nitrogen that has a temperature gradientof 300 to 800° C., while extending it by 2 to 7%. A preferableprocessing temperature is 300 to 800° C. and the stabilized fiber isprocessed under linear temperature gradient conditions. Taking intoconsideration the temperature in the stabilization treatment step, theinitiation temperature is preferably 300° C. or more. If the highesttemperature exceeds 800° C., the fiber that is processed becomes veryfragile and it is difficult to operate the following step. A morepreferable temperature range is 300 to 750° C. The temperature gradientis not particularly limited; however a linear gradient is preferablyemployed.

If the extension rate is less than 2%, orientation of a fibril structurecannot be maintained and the orientation degree along the fiber axis information of a carbon fiber structure will not be sufficient, with theresult that excellent mechanical performance cannot develop. Incontrast, if the extension rate exceeds 7%, a fibril structure itself isbroken, with the result that formation of a carbon fiber structure willbe impaired and a fracture point will change to a defective point. As aresult, a carbon fiber having high strength cannot be obtained. A morepreferable extension rate is 3 to 5%.

In the first carbonization furnace, a preferable heat treatment time is1.0 to 3.0 minutes. If the treatment time is less than 1.0 minute, thetemperature will abruptly increase, and this will be accompanied by asevere decomposition reaction. As a result, a carbon fiber that has highstrength cannot be obtained. If the treatment time exceeds 3.0 minutes,effect of plasticization in the former step will be produced, with theresult that the orientation degree of a crystal will tend to decrease.As a result the mechanical performance of the resultant carbon fiberwill be impaired. A more preferable heat treatment time is 1.2 to 2.5minutes.

Furthermore, the stabilized fiber is subjected to a second carbonizationfurnace that has an inert atmosphere such as nitrogen having atemperature gradient of 1000 to 1600° C. and is treated under heat undertension to obtain a carbon fiber. Furthermore, if necessary, the carbonfiber is optionally subjected to a third carbonization furnace having adesired temperature gradient and is treated with heat in an inertatmosphere under tension.

The temperature for carbonization treatment is determined depending uponthe desired elastic modulus of a carbon fiber. In order to obtain acarbon fiber that has high strength property, the highest temperature ofcarbonization treatment is preferably low. Furthermore, elastic moduluscan be increased by increasing the treatment time. As a result, thehighest temperature can be reduced. Moreover, the gradient oftemperature can be set to change gradually by increasing the treatmenttime. This is effective in suppressing defective point formation. Thetemperature of the second carbonization furnace varies depending uponthe temperature of the first carbonization furnace; however, thetemperature is acceptable as long as it is a temperature of 1000° C. ormore and preferably 1050° C. or more. The temperature gradient is notparticularly limited; however it is preferably set to change linearly.

In the second carbonization furnace, the heat treatment time ispreferably 1.3 to 5.0 minutes and more preferably 2.0 to 4.2 minutes. Inthe heat treatment, the fiber significantly shrinks during processing.Thus, it is important to perform heat treatment under tension.

An extension rate is preferably −6.0 to 0.0%. If the extension rate isless than −6.0%, the orientation of a crystal in the fiber-axisdirection is not satisfactory and sufficient performance cannot beobtained. In contrast, if the extension rate exceeds 0.0%, the structureso far formed itself is broken and defective points are significantlyformed, with the result that the strength is significantly reduced. Morepreferably, the extension rate ranges −5.0% to −1.0%.

Next, the carbon fiber bundle is subjected to surface oxidizationtreatment. Examples of the surface treatment method include knownmethods, i.e., oxidation treatments such as electrolytic oxidation,chemical oxidation and air oxidation. Any one of these methods may beemployed. The electrolytic oxidation treatment industrially widely usedis preferred since surface oxidization treatment can be stablyperformed. In addition, to control the ipa value, which represents apreferable surface treatment state in the present invention, so that theipa value falls within the aforementioned range, performing theelectrolytic oxidation treatment while varying electric quantity is thesimplest method. In this case, even if the electric quantity is thesame, the ipa value significantly varies depending upon the electrolyteand the concentration thereof that is used. In the present invention,electrolytic oxidation treatment can be performed preferably in anaqueous alkaline solution of pH>7 with a carbon fiber as an anode whilesupplying an electric quantity of 10 to 200 coulomb/g. By usingoxidation treatment, the ipa value of 0.05 to 0.25 μA/cm² can beobtained. Examples of electrolyte that is preferably used includeammonium carbonate, ammonium bicarbonate, calcium hydroxide, sodiumhydroxide and potassium hydroxide.

Next, the carbon fiber bundle of the present invention is subjected tosizing treatment. The sizing agent is dissolved in an organic solvent ordispersed in water with the help of an emulsifier to prepare an emulsionsolution. The above preparation is applied to a carbon fiber bundle inaccordance with a roller dip method, a roller contact method, etc.Subsequently, the carbon fiber bundle is dried. In this manner, sizingtreatment can be performed. Note that the amount of the sizing agentattached to the surface of a carbon fiber can be controlled bycontrolling the concentration of the sizing agent solution and thesqueeze amount thereof. Furthermore, drying can be performed by use ofhot air, a hot plate, a heating roller and various infrared heaters.

The most preferable sizing agent composition to be applied to thesurface of the carbon fiber of the present invention is a urethanemodified epoxy resin, which is a reaction product of (a) an epoxy resinthat contains a hydroxy group (hereinafter appropriately referred to asa component (a)), (b) a polyhydroxy compound (hereinafter appropriatelyreferred to as a component (b)) and (c) a diisocyanate that contains anaromatic ring (hereinafter appropriately referred to as a component(c)). Furthermore, a mixture of a urethane modified epoxy resin, whichis a reaction product that is obtained by introducing a larger amount ofcomponent (a) than is required for a reaction system, and component (a)that remains is mentioned.

Moreover, a mixture of a urethane modified epoxy resin obtained by usingan epoxy resin having no hydroxy group (hereinafter appropriatelyreferred to as component (d)) and component (d) is mentioned. Also, amixture of a urethane modified epoxy resin, component (a) and component(d) is mentioned.

An epoxy group has a very strong interaction with an oxygen-containingfunctional group present in a carbon fiber surface and thus can stronglyallow a sizing-agent component to adhere to the carbon fiber surface.Furthermore, since a urethane bonding unit, which is produced from apolyhydroxy compound and a diisocyanate that contains an aromatic ring,is contained, flexibility and strong interaction with a carbon fibersurface due to the polarity of a urethane bond and an aromatic ring canbe imparted. Accordingly, the urethane modified epoxy resin that has anepoxy group and the above urethane bonding unit is a compound that iscapable of strongly bonding to a carbon fiber surface and that hasflexibility. In other words, such a sizing agent composition forms aflexible interface layer that strongly adheres to a carbon fibersurface. Thus, the mechanical performance of the composite materialobtained by impregnating a carbon fiber with a matrix resin and cuingthe resin can be improved.

The component (a) is not particularly limited, and the number of hydroxygroups contained in the component (a) is not limited. For example,glycidol, methyl glycidol, bisphenol type F epoxy resin, bisphenol typeA epoxy resin and oxycarboxylic acid glycidyl ester epoxy resin can beused. A particular preferable resin is a bisphenol type epoxy resin.Since these resins have an aromatic ring, they interact strongly with acarbon fiber surface. Furthermore, in view of heat resistance andrigidity, an epoxy resin having an aromatic ring is often used as thematrix resin in a composite material. This is because compatibility withsuch a matrix resin is excellent.

As component (a), two types or more epoxy resins can be used.

Furthermore, component (b) is preferably constituted of any one fromamong an adduct of bisphenol A with an alkylene oxide, an aliphaticpolyhydroxy compound and a polyhydroxymonocarboxy compound or a mixtureof these. This is because these compounds can soften the aforementionedurethane modified epoxy resins. Specific examples thereof include anadduct of bisphenol A with 4 to 14 mol of ethylene oxide, an adduct ofbisphenol A with 2 to 14 mol of propylene oxide, an adduct of bisphenolA with ethylene oxide-propylene oxide block copolymer, polyethyleneglycol, trimethylolpropane and dimethylolpropionic acid.

Furthermore, component (c) is not particularly limited and a particularpreferable component is toluene diisocyanate or xylene diisocyanate.

Furthermore, an epoxy resin that serves as component (d) is notparticularly limited. Preferably, an epoxy resin having two or moreepoxy groups in a molecule is employed. This is because the surface of acarbon fiber strongly interacts with an epoxy group and thus thesecompounds strongly adhere to the surface. The type of epoxy group is notparticularly limited and a glycidyl type, an alicyclic epoxy group, etc.can be employed. Examples of preferable epoxy resin that can be usedinclude a bisphenol type-F epoxy resin, a bisphenol type-A epoxy resin,a novolak-type epoxy resin, a dicyclopentadiene type epoxy resin(Epiclon HP-7200 series: manufactured by DIC Corporation),tris(hydroxyphenyl)methane-type epoxy resin (Epicoat 1032H60, 1032S50:manufactured by Japan Epoxy Resins Co., Ltd.), DPP Novolak type epoxyresin (Epicoat 157S65, 157S70: manufactured by Japan Epoxy Resins Co.,Ltd.) and bisphenol A alkylene oxide-added epoxy resin.

In manufacturing the mixture that contains component (d), when component(a), component (b) and component (c) are reacted, component (d) may beadded simultaneously with component (a). Alternatively, after completionof the urethanization reaction, component (d) may be added. As a waterdispersion that contains these compounds, HYDRAN N320 (manufactured byDIC) is mentioned.

The carbon fiber bundle of the present invention that has a strandelastic modulus of 250 GPa or more is obtained through a carbonizingprocess performed at relatively high temperature. Accordingly, it isbeneficial that the carbon fiber is obtained from a precursor fiber thatcontains as few impurities such as a metal as possible. The amount ofmetal content in the resultant carbon fiber bundle is preferable small.In particular, the total amount of metal components including alkalinemetal, alkaline earth metal, zinc, iron and aluminum is preferably 50ppm or less. These metals react with carbon, melt or vaporize at atemperature beyond 1000° C., and this constitutes a reason why defectivepoints are formed. Highly strong carbon fiber cannot be manufactured.

EXAMPLES

Now, the present invention will be described in detail by way ofExamples. Note that, performance of a carbon fiber bundle in Exampleswas measured and evaluated in accordance with the following method.

<1. Measurement of Uneven Surface Structure of Single Fiber>

The uneven surface structure can be measured based on the shape of thesurface as follows:

Several single-fibers of a carbon fiber bundle were placed on a samplestand and immobilized at both ends. Furthermore, Dotite was appliedaround the fibers to prepare a measurement sample. A range of 1000 nm inthe circumference direction of a single fiber was scanned by an atomicforce microscope (SPI3700/SPA-300 (trade name) manufactured by SeikoInstruments Inc.) using a cantilever formed of silicon nitride, in anAFM mode while sliding little by little at a distance of 1000 nm in thefiber-axis direction. From the resultant measured image, a low-frequencycomponent was cut off in accordance with a two-dimensional Fouriertransform and thereafter subjected to inverse transformation. From theresultant planar image of a cross-section from which curvature of thesingle fiber was removed, an area range surrounded by a length of 1.0 μmin the fiber-circumference direction and a length of 1.0 μm in thefiber-axis direction was selected and the difference in height betweenthe highest portion and the lowest portion was read out, and Ra isobtained by making a calculation in accordance with the followingexpression (2).

Ra={1/(Lx×Ly)}·∫^(Ly) ₀∫^(LX) ₀ |f(x,y)|dxdy  (2)

“Center surface”: A plane which is parallel to the plane that has aminimum deviation of the height from the place to the actual surface andthat divides the actual surface equally into two based on volume. Inother words, the portion that is surrounded by the plane and the actualsurface, volumes V1 and V2 that correspond to both side portions of theplane are equal to each other.

“f(x, y)”: Difference in height between the actual surface and thecenter surface,

“Lx, Ly”: Size of the XY plane.

Furthermore, when an atomic force microscope was used to takemeasurements, the presence or absence of an uneven structure of 0.6 μmor more in length was checked and the length of an uneven structure of300 nm or less in length was measured.

<2. Evaluation of a Cross-Sectional Shape of Single Fiber>

The ratio of the major axis and the minor axis (major axis/minor axis)of the cross section of single fibers that constitute a carbon fiberbundle was determined as follows:

A carbon fiber bundle for measurement was passed through a tube formedof a vinyl chloride resin that has an inner diameter of 1 mm. The tubewas cut in the shape of a circle by a knife to prepare samples.Subsequently, each sample was allowed to adhere to a SEM sample stand sothat the cross-section faced up; Au was deposited in a thickness ofabout 10 nm by sputtering; and a fiber cross-section was observed by ascanning electron microscope (product name: XL20, manufactured byPhilips) under the following conditions: an acceleration voltage of 7.00kV and a migration distance of 31 mm to measure the major axis and minoraxis of the cross section of the single fiber.

<3. Evaluation of Strand Physical Properties of Carbon Fiber Bundle>

Preparation of a strand test sample of a carbon fiber bundle impregnatedwith a resin and measurement of the strength of the sample wereperformed in accordance with JIS R7601. However, the elastic modulus wascalculated in the range of strain in accordance with ASTM.

<4. Measurement of Knot Tenacity of Carbon Fiber Bundle>

Knot tenacity was measured as follows:

To both ends of a carbon fiber bundle of 150 mm in length, a gripportion of 25 mm in length was attached to prepare a test sample. Inpreparing the test sample, a load of 0.1×10⁻³ N/denier was applied tounidirectionally arrange carbon fiber bundles. In the test sample, asingle knot was formed almost in the center. Tension was performed at acrosshead rate of 100 mm/min. Twelve bundles were used for a test. Thesmallest value and the largest value were removed and the average valueof 10 bundles was obtained and used as a measurement value.

<5. Evaluation of “Surface Energy of Surface Formed by Fracture” ofCarbon Fiber Bundle>

A single carbon fiber was cut into pieces in the length of 20 cm. Eachof the single-fiber pieces was allowed to adhere and immobilize onto amount for a tensile test for a single fiber of 10-mm sample length shownin JIS R7606. An extra portion extruded from the mount was cut andremoved. In this manner, samples were prepared.

Next, these samples that were fixed to the mount were irradiated with alaser to obtain a hemispherical defect. As a laser interface system,MicroPoint (pulse energy 300 μJ) manufactured by Photonic InstrumentsCo., Ltd. were used. As the optical microscope required for converginglaser light, ECLIPSE LV100 manufactured by Nikon was used. The aperturestop of the optical microscope was set to a minimum and an object lenswas set at 100 times. Under the conditions, a laser (one pulse) having awavelength of 435 nm, the intensity of which was attenuated by 10% by anattenuator, was applied to the center of a sample in the fiber-axisdirection and in the perpendicular direction of the fiber axis to form ahemispherical defect.

To avoid shrinkage break of a sample carbon fiber, the sample that wasattached to a mount was further sandwiched by films and the space withinthe films was filled with viscous liquid and then subjected to a tensiletest. To describe more specifically, a film of about 5 mm in width andabout 15 mm in length was prepared and allowed to adhere to the upperportion of both surfaces of the sample mount with an adhesive andsandwiched so as to wrap the sample including the mount. The spacewithin the film was filled with an aqueous glycerin solution(glycerin:water=1:2). A tensile test was performed at a tension rate of0.5 mm/min to measure the breaking load.

Next, a sample of a pair of pieces, which was divided into two pieces inthe tensile test, was removed from the mount, carefully washed withwater and naturally dried. Subsequently, a sample piece was immobilizedonto a SEM sample stand with carbon paste so that the fracture was in aface up position to enable the preparation of a sample for SEMobservation. The fracture surface of the obtained sample for SEMobservation was observed by SEM, i.e., JSM6060 (acceleration voltage: 10to 15 kV, magnification: 10000 to 15000) manufactured by JEOL Ltd.

The obtained SEM image was imported into a personal computer andanalyzed by image analyzing software. In this manner, the size of ahemispherical defect and the cross-sectional area of a fiber weremeasured.

Next, breaking strength (a) (breaking load/fiber cross-sectional area)and the size of a hemispherical defect (C) were plotted. The slope ofthe data was obtained.

σ=(2E/πC)^(1/2)×(surface energy of surface formed byfracture)^(1/2)  (1)

In accordance with Equation (1), surface energy of surface formed byfracture was obtained from the slope and ultrasonic elastic modulus (E)of a carbon fiber bundle.

<6. Measurement of Ipa of Carbon Fiber Bundle>

An Ipa value was measured by the following method.

An electrolyte is adjusted to pH3 with a 5% aqueous phosphoric acidsolution and bubbled with nitrogen to eliminate the effect of dissolvedoxygen. A sample carbon fiber was dipped in the electrolyte as one ofthe electrodes, whereas, a platinum electrode having a sufficientsurface area is used as the other electrode. Herein, as a referenceelectrode, an Ag/AgCl electrode was employed. A sample was formed of a12000-filament tow having a length of 50 mm. The potential to be appliedbetween the carbon fiber electrode and the platinum electrode rangesfrom −0.2 V from +0.8 V and a scanning speed thereof was set to be 2.0mV/sec. A current-voltage curve was drawn by an X-Y recorder. Aftersweeping was performed three times or more to obtain a stable curve, acurrent value i was read with a potential of +0.4 V applied to theAg/AgCl reference electrode that was used as a reference potential, andan ipa value was calculated in accordance with the following expression(3).

ipa=1(μA)/sample length(cm)×{4π×weight per unit area(g/cm)×number offilaments/density(g/cm³)}^(1/2)  (3)

The apparent surface area was calculated from sample length, sampledensity obtained by the method described in JIS R7601 and mass per unitarea. Current value i was divided by the obtained value to obtain an ipavalue. The measurement was performed by cyclic voltammetry analyzer TypeP-1100 manufactured by Yanagimoto Mfg. Co., Ltd.

<7. Measurement of Si Amount of Carbon Fiber Bundle>

A sample of a carbon fiber bundle was placed in a platinum cruciblewhose tare was known and calcified in a muffle furnace of 600 to 700° C.The mass thereof was measured to obtain ash content. Subsequently, apredetermined amount of sodium carbonate was added, and the mixture wasmelted by a burner and dissolved with deionized water in a 50 ml polymeasuring flask. The sample was measured by ICP emission spectrometry todetermine the amount of Si.

Production Examples 1 to 7 of Precursor Fiber Bundle Precursor Fiber (1)

An acrylonitrile based polymer containing acrylonitrile (98 mass %) andmethacrylic acid (2 mass %) was dissolved in dimethylformamide toprepare a 23.5 mass % dope.

The dope was ejected from a spinneret having 0.15 mm diameter with 2000ejection holes arranged therein in a dry-wet spinning manner. To explainmore specifically, the dope was ejected into the air, passed through aspace of about 5 mm and then solidified in a congealed liquid filledwith an aqueous solution containing 79.0 mass % dimethyl formamide andcontrolled at a temperature of 10° C. to obtain a coagulated fiber.Subsequently, the coagulated fiber was drawn (1.1 times) in the air anddrawn (2.5 times) in a drawing tank filled with an aqueous solutioncontaining 35 mass % dimethyl formamide and controlled at 60° C. Afterthe drawing process, the fiber bundle in which a solvent was present waswashed with clean water and then drawn (1.4 times) in hot water of 95°C. Sequentially, to the fiber bundle, an oil solution containing anamino modified silicone as a main component was attached so as to apply1.1 mass %, dried and densified. After the drying and densificationstep, the fiber bundle was drawn (2.6 times) between heated rolls tofurther improve the orientation degree and densification. Thereafter,the bundle was wound up to obtain an acrylonitrile-based precursor fiberbundle. The denier of the fiber was 0.77 dtex.

Precursor Fiber (2)

Precursor fiber bundle (2) was obtained under the same conditions as inprecursor fiber bundle (1) except that the draw ratio before washingwith water was set to be 2.9 times and the draw ratio in hot water afterwashing was set to be 1.2 times.

Precursor Fiber (3)

Precursor fiber bundle (3) was obtained under the same conditions as inprecursor fiber bundle (2) except that the denier of the precursor fiberwas set to be 0.67 dtex.

Precursor Fiber (4)

Precursor fiber bundle (4) was obtained under the same conditions as inprecursor fiber bundle (2) except that the denier of the precursor fiberwas set to be 0.90 dtex.

Precursor Fiber (5)

Precursor fiber bundle (5) was obtained under the same conditions as inprecursor fiber bundle (1) except that the draw ratio before washingwith water was set to be 4.1 times, the draw ratio in hot water afterwashing was set to be 0.9 times and drawing that was performed betweenheated rolls was set to be 2.4 times.

Precursor Fiber (6)

Precursor fiber bundle (6) was obtained under the same conditions as inprecursor fiber bundle (1) except that the draw ratio before washingwith water was set to be 1.9 times and the draw ratio in hot water afterwashing was set to be 2.0 times.

Precursor Fiber (7)

Precursor fiber bundle (7) was obtained under the same conditions as inprecursor fiber bundle (2) except that the denier of the precursor fiberwas set to be 1.0 dtex.

The manufacturing conditions for precursor fiber bundles (1) to (7) areshown in Table 1.

TABLE 1 Spinning conditions Precursor Coagulation bath In the air Drawtank Hot water Dry heat fiber Precursor Temperature Concentration DrawTemperature Concentration Draw Draw drawing Denier fiber (° C.) (%)ratio (° C.) (%) ratio ratio Ratio dtex Precursor 10 79.0 1.1 60 35 2.51.4 2.6 0.77 (1) Precursor 10 79.0 1.1 60 35 2.9 1.2 2.6 0.77 (2)Precursor 10 79.0 1.1 60 35 2.9 1.2 2.6 0.67 (3) Precursor 10 79.0 1.160 35 2.9 1.2 2.6 0.90 (4) Precursor 10 79.0 1.1 60 35 4.1 0.99 2.4 0.77(5) Precursor 10 79.0 1.1 60 35 1.9 2.0 2.4 0.77 (6) Precursor 10 79.01.1 60 35 2.9 1.2 2.6 1.00 (7) Precursor 5 77.0 1.3 60 0 2.0 2.0 1.90.77 (8)

Examples 1 to 7, Comparative Examples 1 to 4 Preparation of Carbon FiberBundle

A plurality of precursor fiber bundles (1), (2), (3), (4), (5), (6) or(7) were arranged in parallel and introduced in an oven forstabilization. Air heated to 220 to 280° C. was sprayed to the precursorfiber bundles to perform stabilization treatment. In this manner,stabilized fiber bundles having a density of 1.345 g/cm³ were obtained.The extension rate was set to be 6% and stabilization treatment time was70 minutes.

Next, the stabilized fiber bundles were passed through a firstcarbonization furnace having a temperature gradient of 300 to 700° C. innitrogen while the extension rate was increased by 4.5%. The temperaturegradient was set so as to change linearly. The treatment time was 2.0minutes.

Furthermore, heat treatment was performed using a second carbonizationfurnace having a nitrogen atmosphere in which the temperature gradientof 1000 to 1600° C. could be set, at the predetermined temperaturesshown in Table 2 or Table 3. Subsequently, heat treatment was performedusing a third carbonization furnace having a nitrogen atmosphere inwhich the temperature gradient of 1200 to 2400° C. could be set, at thepredetermined temperatures shown in Table 2 or Table 3 to obtain carbonfiber bundles. The total extension rate of the fiber bundles through thetreatments in the second carbonization furnace and third carbonizationfurnace was −4.0%, and the treatment time in total was set to be 3.5minutes.

Subsequently, the fiber bundles were fed to a 10 mass % aqueous ammoniumbicarbonate solution. Current was supplied between a carbon fiber bundleserving as an anode and a counter pole so as to obtain a quantity ofelectricity of 40 coulombs per carbon fiber (1 g) to be treated. Then,the fiber bundles were washed with warm water of 90° C. and dried.

Next, Hydran N320 (hereinafter referred to as “sizing agent 1”) wasattached onto the fiber bundles in the amount of 0.5 mass % and wound bya bobbin to obtain a carbon fiber bundle.

(Preparation of UniDirectional Prepreg)

Onto a mold-releasing paper coated with epoxy resin #410 (that can beused at 180° C.) [manufactured by Mitsubishi Rayon Co., Ltd.] in the Bstage, 156 carbon fiber bundles released from a bobbin were arranged inparallel and passed through a heat compression roller. In this manner,the carbon fiber bundles were impregnated with epoxy resin. A protectingfilm was laminated on the resultant bundles to prepare a prepregarranged in a unidirection (hereinafter referred to as a “UD prepreg”)having a resin content of about 33 mass %, a carbon fiber mass per unitarea of 125 g/m² and a width of 500 mm.

(Molding of a Laminate Board and Evaluation of Mechanical Performance)

A laminate board was prepared by using the UD prepreg and the tensilestrength of the laminate board at angle 0° was evaluated by theevaluation method in accordance with ASTM D3039.

The manufacturing conditions for a carbon fiber bundle and theevaluation results are shown in Table 2 and Table 3.

Note that it was confirmed that in any of Examples, a single fiber doesnot have an uneven surface structure that has a length of 0.6 μm or moreand that extends in the longitudinal direction of a fiber but has amicroscopic uneven structure that has a length of 300 nm or less.

TABLE 2 Carbon fiber bundle Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Example 8 Type of precursor fiberPrecursor Precursor Precursor Precursor Precursor Precursor PrecursorPrecursor bundle (1) (2) (3) (4) (2) (2) (2) (2) Conditions of second1250-1300 1250-1300 1250-1300 1250-1300 1050-1250 1100-1301 1200-15001050-1250 carbonization furnace (° C.) Conditions of third 1350-15501350-1550 1350-1550 1350-1550 1250-1450 1300-1501 1550-1800 —carbonization furnace (° C.) Difference in height (Rp − v) 12 15 14 1615 15 15 15 (nm) Ra (nm) 3 4 3 4 4 4 4 4 Major axis/minor axis of 1.0051.005 1.005 1.005 1.005 1.005 1.005 1.005 cross-section Weight per unitarea of 0.035 0.035 0.030 0.041 0.035 0.035 0.035 0.035 single fiber(mg/m) Strand strength (MPa) 6050 6300 6400 6100 6700 6500 6050 6350Strand elastic modulus 330 335 345 325 305 325 355 260 (GPa) Knottenacity (N/mm²) 950 1040 1100 1000 1200 1100 910 1250 Surface energy ofsurface 32 33 34 33 35 34 30 36 formed by fracture (N/m) iPa (μA/cm²)0.15 0.15 0.15 0.15 0.17 0.16 0.09 0.18 O1S/C1S 0.09 0.09 0.09 0.09 0.110.1 0.07 0.13 Si amount (ppm) 150 120 110 130 120 120 125 120 Metalcontent (ppm) 40 40 40 40 40 40 40 40 Tensile strength of 3000 3150 31803100 3400 3300 3020 3240 laminate board at 0° C. (in terms of 60 vol %)MPa

TABLE 3 Comparative Comparative Comparative Comparative ComparativeCarbon fiber bundle Example 1 Example 2 Example 3 Example 4 Example 5Type of precursor fiber bundle Precursor (5) Precursor (6) Precursor (7)Precursor (6) Precursor (8) Conditions of second carbonization 1250-13001250-1300 1250-1300 1200-1500 1250-1300 furnace (° C.) Conditions ofthird carbonization 1350-1550 1350-1550 1350-1550 1550-1800 1350-1550furnace (° C.) Difference in height(Rp − v) (nm) 29 13 23 13 8 Ra (nm) 83 6 3 2 Major axis/minor axis of cross-section 1.005 1.005 1.005 1.0051.005 Weight per unit area of single fiber (mg/m) 0.035 0.035 0.0450.045 0.045 Strand strength (MPa) 5400 5700 5750 5200 5950 Strandelastic modulus (GPa) 330 330 315 355 325 Knot tenacity (N/mm²) 650 750850 630 790 Surface energy of surface formed by 29 30 30 28 29 fracture(N/m) iPa (μA/cm²) 0.18 0.15 0.15 0.09 0.15 O1S/C1S 0.12 0.10 0.09 0.070.11 Si amount (ppm) 300 220 160 210 230 Metal content (ppm) 40 40 40 4040 Tensile strength of laminate board at 0° C. 2650 2700 2700 2400 2850(in terms of 60 vol %) MPa

Production Example 8 of Precursor Fiber Bundle

A dope, which was prepared in the same manner as in Production Example1, was ejected from a spinneret having 0.13 mm diameter with 2000 ofejection holes arranged therein in a dry-wet spinning manner. To explainmore specifically, the dope was ejected into the air, passed through aspace of about 5 mm and then solidified in a congealed liquid filledwith an aqueous solution containing 77.0 mass % dimethyl formamide andcontrolled at a temperature of 5° C. to obtain a coagulated fiber.Subsequently, the coagulated fiber was drawn (1.3 times) in the air anddrawn (2.0 times) in a drawing tank filled with an aqueous solutioncontrolled at 60° C. After the drawing process, the fiber bundle inwhich a solvent was present was washed with clean water and then drawn(2.0 times) in hot water of 95° C. Sequentially, to the fiber bundle, anoil solution containing an amino modified silicone as a main componentwas attached so as to apply 1.0 mass %, dried and densified. After thedrying and densification step, the fiber bundle was drawn (1.9 times)between heated rolls to further improve the orientation degree anddensification. Thereafter, the bundle was wound up to obtain theprecursor fiber bundle. The denier of the fiber was 0.77 dtex.

Example 8

A carbon fiber bundle was prepared under the same carbonizing conditionsas in Example 5 except that the third carbonization furnace was notused. Furthermore, a laminate board was prepared in the same manner andmechanical performance was evaluated to obtain the results shown inTable 2. Note that, it was confirmed that the single fiber has no unevensurface structure having a length of 0.6 μm or more and extending in thelongitudinal direction of the fiber and has an uneven microscopicstructure having a length of 300 nm or less.

Examples 9 to 11, Comparative Examples 6 to 8

Carbon fiber bundles were obtained in the same manner as in Example 2except that the carbonizing conditions were changed. The evaluationresults are shown in Table 4. Note that, in any one of Examples, it wasconfirmed that the single fiber has no uneven surface structure having alength of 0.6 μm or more and extending in the longitudinal direction ofthe fiber and has an uneven microscopic structure having a length of 300nm or less.

TABLE 4 Comparative Comparative Comparative Carbon fiber bundle Example9 Example 6 Example 10 Example 7 Example 11 Example 8 Conditions changedfrom Stabilized Stabilized First First Second, third Second, thirdExample 2 fiber extension fiber extension carbonated carbonatedcarbonated carbonated rate rate fiber extension fiber extension fiberextension fiber extension rate rate rate rate 8.0% 10.5% 6.5% 7.2% −5.2%−6.5% Difference in height(Rp − v) (nm) 13 12 14 13 15 16 Ra (nm) 4 4 44 4 4 Major axis/minor axis of cross- 1.005 1.005 1.005 1.005 1.0051.005 section Weight per unit area of single fiber 0.034 0.033 0.0340.033 0.035 0.035 (mg/m) Strand strength (MPa) 6000 5300 5900 5400 60505500 Strand elastic modulus (GPa) 338 340 336 336 327 310 Surface energyof surface formed 990 700 970 750 990 800 by fracture (N/m) Knottenacity (N/mm²) 31 27 30 28 31 30 iPa (μA/cm²) 0.15 0.15 0.15 0.15 0.150.15 O1S/C1S 0.09 0.09 0.09 0.09 0.09 0.09 Si amount (ppm) 120 120 120120 120 120 Metal content (ppm) 40 40 40 40 40 40 Tensile strength oflaminate board at 3000 2550 2950 2500 2980 2600 0° C. (in terms of 60vol %) MPa

Examples 12 and 13

Carbon fiber bundles were obtained in the same manner as in Example 5except that the surface treatment conditions were changed. Theevaluation results are shown in Table 5. Note that in any of theExamples, it was confirmed that the single fiber has no uneven surfacestructure having a length of 0.6 μm or more and extending in thelongitudinal direction of the fiber and has an uneven microscopicstructure having a length of 300 nm or less.

Examples 14 to 16

Carbon fiber bundles were obtained in the same manner as in Example 5except that the types and amounts attached of sizing agents werechanged. The evaluation results are shown in Table 5. Note that in anyone of the Examples, it was confirmed that the single fiber has nouneven surface structure having a length of 0.6 μm or more and extendingin the longitudinal direction of the fiber and has an uneven microscopicstructure having a length of 300 nm or less.

Note that, sizing agent 2, sizing agent 3 and sizing agent 4 wereprepared as follows:

(Sizing Agent 2)

“Epicoat 828” (80 parts by mass) manufactured by Japan Epoxy Resins Co.,Ltd. as a main agent and “Pluronic F88” (20 parts by mass) manufacturedby ADEKA Corporation as an emulsifier were blended to obtain an aqueousdispersion by phase-transfer emulsification.

(Sizing Agent 3)

In a flask, polyol (3.2 mol) consisting of 1.8 mol of an adduct ofbisphenol A with 8 mol of propylene oxide, 0.8 mol of trimethylolpropaneand 0.6 mol of dimethyloipropionic acid were placed. Furthermore, 0.5 gof 2,6-di(t-butyl) 4-methylphenol (BHT) as a reaction inhibitor and 0.2g of dibutyltin dilaurate as a reaction catalyst were added and stirreduntil a homogeneous mixture was obtained. Herein, if necessary, methylethyl ketone was added as a viscosity moderator. To the mixturehomogeneously dissolved, metaxylene diisocyanate (3.4 mol) was addeddropwise and polymerization of a urethane prepolymer was performed whilestirring at a reaction temperature of 50° C. for a reaction time of 2hours. Then, Epicoat 834 (manufactured by JER) (0.25 mol) was added andreacted with an isocyanate group at an end of the urethane prepolymer toobtain an epoxy modified urethane resin.

The epoxy modified urethane resin (90 parts by mass) and “Pluronic F88”(10 parts by mass) manufactured by ADEKA Corporation as an emulsifierwere blended to prepare an aqueous dispersion.

(Sizing Agent 4)

To a flask, polyethylene glycol 400 (2.5 mol) and Epicoat 834(manufactured by JER) (0.7 mol) were placed and further2,6-di(t-butyl)-4-methylphenol (BHT) (0.25 g) as a reaction inhibitor,and dibutyl tin dilaurate (0.1 g) as a reaction catalyst were added. Themixture was stirred until it became homogeneous. Herein, if necessary,methyl ethyl ketone was added as a viscosity moderator. To the mixturehomogeneously dissolved, a metaxylene diisocyanate (2.7 mol) was addeddropwise. The reaction was performed at a temperature of 40° C. for 2hours while stirring to obtain an epoxy modified urethane resin.

The epoxy modified urethane resin (80 parts by mass) and Pluronic F88(20 parts by mass) manufactured by ADEKA Corporation as an emulsifierwere mixed to prepare an aqueous dispersion.

TABLE 5 Carbon fiber bundle Example 12 Example 13 Example 14 Example 15Example 16 Conditions changed from Example 5 Surface Surface Sizingagent Sizing agent Sizing agent treatment treatment 8% Ammonium AmmoniumSizing agent 2 Sizing agent 3 Sizing agent 4 carbonate bicarbonate(amount (amount (amount 20 coulomb/g 200 coulomb/g attached, 0.4attached, 0.4 attached, 0.4 mass %) mass %) mass %) Strand strength(MPa) 6,400 6,700 6,700 6,700 6,700 iPa (μA/cm²) 0.40 0.24 0.17 0.170.17 O1S/C1S 0.19 0.16 0.11 0.11 0.11 Tensile strength of laminate boardat 3000 3280 3050 3410 3350 0° C. (in terms of 60 vol %) MPa

INDUSTRIAL APPLICABILITY

The carbon fiber bundle of the present invention can be used as aconstructional material for airplanes and high speed moving bodies.

1. A carbon fiber bundle formed of the single carbon fiber: each singlecarbon fiber having has no uneven surface structure that has a length of0.6 μm or more and that extends in the longitudinal direction of thesingle fiber, each single carbon fiber having an uneven structure inwhich the difference in height (Rp−v) between a highest portion and alowest portion of the surface of the single fiber is 5 to 25 nm, and inwhich an average roughness Ra is 2 to 6 nm, and each of which has aratio of 1.00 to 1.01 of a major axis to a minor axis (major axis/minoraxis) of a cross-section of the single fiber; wherein a mass of thesingle fiber per unit length falls within the range of 0.030 to 0.042mg/m; a strand strength is 5900 MPa or more; a strand elastic modulusmeasured by the ASTM method is 250 to 380 GPa; and a knot tenacity is900 N/mm² or more.
 2. A carbon fiber bundle formed of the single carbonfiber: each single carbon fiber having no uneven surface structure thathas a length of 0.6 μm or more and that extends in the longitudinaldirection of the single fiber, each single carbon fiber having an unevenstructure that has a length of 300 nm or less in which the difference inheight (Rp−v) between a highest portion and a lowest portion of thesurface of the single fiber is 5 to 25 nm, and in which an averageroughness Ra is 2 to 6 nm, and each of which has a ratio of 1.00 to 1.01of a major axis to a minor axis (major axis/minor axis) of across-section of the single fiber; wherein a mass of the single fiberper unit length falls within the range of 0.030 to 0.042 mg/m; a strandstrength is 5900 MPa or more; a strand elastic modulus measured by theASTM method is 250 to 380 GPa; and a knot tenacity is 900 N/mm² or more.3. The carbon fiber bundle according to claim 1 or 2, wherein ahemispherical defect having a size within a predetermined range isformed on a single fiber surface by a laser, the fiber is broken at thehemispherical defective site by a tensile test, and “surface energy ofsurface formed by fracture” obtained from breaking strength of the fiberand the size of the hemispherical defect in accordance with the GriffithEquation (1) is 30 N/m or more, wherein σ is a breaking strength; E isan ultrasonic elastic modulus of a carbon fiber bundle; and C is a sizeof a hemispherical defect.σ=(2E/πC)^(1/2)×(surface energy of surface formed byfracture)^(1/2)  (1)
 4. The carbon fiber bundle according to any ofclaims 1 to 3, wherein an ipa value obtained by an electrochemicalmeasuring method (cyclic voltammetry) is 0.05 to 0.25 μA/cm², and anamount of an oxygen-containing functional group (O1S/C1S) in a carbonfiber surface obtained by X-ray photoelectron spectroscopy falls withinthe range of 0.05 to 0.15.
 5. The carbon fiber bundle according to anyof claims 1 to 4, wherein a Si amount measured by ICP emissionspectrometry is 200 ppm or less.
 6. The carbon fiber bundle according toany of claims 1 to 5, sized by a sizing agent composition comprising aurethane modified epoxy resin, which is a reaction product of (a) anepoxy resin having a hydroxy group, (b) a polyhydroxy compound and (c) adiisocyanate containing an aromatic ring, or sized by a sizing agentcomposition comprising a mixture of the urethane modified epoxy resinand (a) an epoxy resin having a hydroxy group and/or (d) an epoxy resinhaving no hydroxy group.
 7. The carbon fiber bundle according to claim6, wherein (a) the epoxy resin having a hydroxy group is a bisphenoltype epoxy resin.
 8. The carbon fiber bundle according to claim 6 or 7,wherein (b) the polyhydroxy compound is any one from among an adduct ofa bisphenol-A with an alkylene oxide, an aliphatic polyhydroxy compoundand a polyhydroxy monocarboxy compound or a mixture thereof.
 9. Thecarbon fiber bundle according to any of claims 6 to 8, wherein (c) thediisocyanate containing an aromatic ring is toluene diisocyanate orxylene diisocyanate.
 10. The carbon fiber bundle according to any ofclaims 1 to 9, wherein a total content of metals including an alkalinemetal, an alkaline earth metal, zinc, iron and aluminum is 50 ppm orless.